A set of standards have been promulgated by the Institute of Electrical and Electronic Engineers (IEEE) which specify the port characteristics for a coaxial media interface to Data Terminal Equipment (DTE). The IEEE 802.3 Local Area Network Standard employs a "collision detection" method to indicate the presence of more than one station attempting to transmit data simultaneously on the network.
The transmitted data "packet" is composed of both alternating current (AC) and direct current (DC) components. When two or more stations try to transmit data at the same time, the data packets will be corrupted. For example, the AC signal on the coaxial medium can vary a great deal, and may even phase cancel, depending upon the phase relation and the bit pattern. However, the DC component will always be additive. It is this shift in the DC value that is used to indicate the presence of a collision on the network.
The IEEE 802.3 standard makes certain recommendations for the "no detect" and "must detect" levels on the network. They are based on calculations made for both 10 Base 5--Ethernet and 10 Base 2--Cheapernet specifications, namely a 5 ohm DC loop resistance and a 10 ohm DC loop system, respectively. For longer networks the DC loop resistances are larger than these values and the collision detect window shrinks until it is impossible to distinguish the two conditions. These considerations lead to a 150 millivolt (mV) to 200 mV collision detect window.
In addition, the IEEE 802.3 standard requires that a collision indication should occur within nine bit times of the actual collision on the coaxial medium. For a 10 megabit per second (Mbps) data stream this is 900 nanoseconds (ns). For a collision detection circuit, which will include a low-pass filter and logic gates, the total delay in the collision detect path must not exceed 900 ns, which places a premium on limiting low-pass filter response and gate delays. The AC component of the signal in which the collision detection circuit must operate is approximately 2 volts peak-to-peak at 10 Mbps.
The characteristics of the collision detection circuit then are:
(1) it shall provide large attenuation to the 10 Mbps data, i.e., ripple must be well controlled;
(2) it shall meet the delay requirements;
(3) it shall not indicate a false collision in the presence of only one transmitting station, i.e., its step response must not have appreciable overshoot;
(4) it should meet the IEEE collision detection standards over a normal range of variation in circuit component parameters, as such parameters vary due to process and temperature.
The prior art has employed discrete components, such as inductors, that require careful trimming adjustments in the field. Furthermore, individual component values were difficult to control when constructing the circuitry out of discrete components, and component matching was difficult to achieve. This made such circuits highly sensitive to parameter variation.